(Meth)acrylic esters of polyhydric alcohols, more particularly from the group of the dihydric to hexahydric aliphatic saturated alcohols, and their oxalkylation products, are finding increasing importance as high-reactivity ingredients in radiation-curing systems. Such polyfunctional (meth)acrylic esters can be used, for example, as coatings raw materials for electron beam curing or as an ingredient of UV-curing printing inks or corresponding coating materials, filling compositions, molding compositions or casting compositions, and also in adhesives, more particularly those which cure anaerobically. Their preparation, however, is not without problems. The requirement in particular is for colorless products with a low acid number and high storage stability, which also have virtually no inherent odor. Distillative purification of the (meth)acrylic esters of the type in question here is generally ruled out by their high molecular weight and their high reactivity. The products are therefore to be obtained directly, as very largely colorless reaction products, from the esterification Implementing the esterification reaction requires the accompanying use of high-activity inhibitors, which in turn do not trigger any unwanted secondary reactions, such as discolorations, for example.
The predominant industrial production process for the direct esterification of (meth)acrylic acid with hydroxy compounds is based on the use of volatile organic solvents as a liquid reaction medium, also known as solvent operation. Suitable volatile organic solvents are, for example, toluene, cyclohexane, methylcyclohexane or n-heptane, which, moreover, are utilized as an azeotropic entrainer for the continuous removal of the resultant water of reaction from the reaction mixture and are removed by distillation after the end of reaction. Corresponding process descriptions are found in U.S. Pat. No. 6,838,515 and EP-A-127,766, for example. In spite of the distillative removal of the volatile organic solvents after the end of reaction, however, monomer and oligomer (meth)acrylates prepared in this way always have residual solvent traces, the amounts of which vary in the range of 50-10 000 ppm.
Since the use of organic solvents is being regulated to increasing degrees due to environmental protection considerations, there is an increasing need for an alternative production process, in which the direct esterification of (meth)acrylic acid with hydroxy compounds may be carried out in (meth)acrylic acid itself as the liquid reaction medium, without use of volatile organic solvents (also known as solvent-free operation). The water of reaction formed is removed from the reaction mixture by distillation, in the form of a water/(meth)acrylic acid mixture. Corresponding processes are described in EP-B-449 919 and EP-B-1 204 472, for example. In this way, completely solvent-free monomer and oligomer (meth)acrylates can be prepared, and are used in applications including those where traces of volatile organic solvents may produce an unwanted inherent odor, such as in the packaging sector, for example.
Both production technologies have been subject to continuous ongoing development over the course of the years, with the focal points being on optimizing the throughput and minimizing the use of raw materials. The resulting purity of the monomer and oligomer (meth)acrylates prepared in this way, in contrast, has been paid no great attention to date. The purity, in the relevant technical literature, is generally understood to be the total amount of fully and partially (meth)acrylated species of substance in the end product, and is calculated from the residual amount of unreacted hydroxy compounds and situated, accordingly, at usually more than 97% (GC area-percent).
Strictly speaking, however, this widespread definition of the concept of purity says nothing about the amount that is actually present of the desired, fully (meth)acrylated species of substance in the end product, and is therefore unsatisfactory. In the context of the present invention, therefore, “purity” means the amount of fully (meth)acrylated species of substance in (meth)acrylic esters. Fully (meth)acrylated species of substance are understood to be those in which all of the OH groups of the alcohol component of the (meth)acrylic ester are present in esterified form.
This definition of the term purity, which applies strictly in the context of the present invention, is particularly important in view in particular of the fact that, for polyfunctional monomer and oligomer (meth)acrylates, the identification and classification of substances under REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) is exclusively via CAS numbers, with corresponding purity requirements related to the respective fully (meth)acrylated species of substance (see ECHA publication “Guidance for Identification and Naming of Substances under REACH—June 2007”). For instance, for what REACH calls “mono-constituent substances”, which include, for example, 1,6-hexanediol diacrylate, tripropylene glycol diacrylate and trimethylolpropane triacrylate, for example, purities of at least 80%—based on the desired, fully acrylated species of substance—in the end product are required.
From the technical literature it is known that, for example, polyfunctional monomer acrylates such as 1,6-hexanediol diacrylate, tripropylene glycol diacrylate, trimethylolpropane triacrylate and pentaerythritol tetraacrylate constitute mixtures of species of substance having different degrees of acrylation with other by-products, in which the desired, fully acrylated species of substance is only part of the mixture (cf. R. H. Hall, F. P. B. Van Der Maeden, A. C. C. M. Willemsen, Spec. Chem., 7, 56-64 (1987) and M. Matsunaga, Y. Matsushima, H. Ohtani, S. Tsuge, Anal. Sci., 17, 1295-1299 (2001)).
Even more complex compositions are a characteristic of those polyfunctional monomer and oligomer (meth)acrylates that are based on polyhydroxy compounds alkoxylated with ethylene oxide and/or propylene oxide. As an inevitable concomitant of the process, such products always have a distribution curve with different degrees of alkoxylation around a middle value, and so the number of possible species of substance (meth)acrylated fully and partially is not only dependent on the number of available hydroxyl groups but is also connected, on a multiplicative basis, with the number of differently alkoxylated polyhydroxy species.
Corresponding investigations into the composition of such polyfunctional, alkoxylated monomer and oligomer acrylates, such as ethoxylated 1,6-hexanediol diacrylate, propoxylated neopentylglycol diacrylate, ethoxylated trimethylolpropane triacrylate, propoxylated glycerol triacrylate and polypropylene glycol diacrylate, are found in T. Marek, U. Gröllman, DIC Technical Review, No. 5, 85-93 (1999), M. Matsunaga, Y. Matsushima, H. Yokoi, H. Ohtani, S. Tsuge, Anal. Sci., 18, 277-281 (2002) and S. J. Yoo, G. V. Pace, B. K. Khoo, J. Lech, T. G. Hartman, RadTech Report, May/June, 60-68 (2004).
The acidically catalyzed, direct esterification of (meth)acrylic acid with monohydroxy compounds can be considered in a first approximation still to be a simple phase equilibrium reaction, in which the phase equilibrium can be shifted almost entirely to the product side by the continuous removal of the water of reaction from the reaction mixture, via the appropriate choice of temperature, pressure and reaction time.

The end product generally contains the desired, fully (meth)acrylated species of substance in purities of greater than 97%.
For polyhydroxy compounds, in contrast, the profile of the acidically catalyzed, direct esterification of (meth)acrylic acid is substantially more complex, as described in L. -D. Shiau, T. -R. Ling, D. -S. Tseng, Chem. Eng. Comm , 179, 133-148 (2000), since different phase equilibrium reactions, coupled with one another, take place alongside one another, and may also result, depending on reaction regime, in substantial amounts of partially (meth)acrylated monomer or oligomer (meth)acrylates in the end product.

Furthermore, as set out by R. H. Hall, F. P. B. Van Der Maeden, A. C. C. M. Willemsen, Spec. Chem., 7, 56-64 (1987), these partially (meth)acrylated monomer or oligomer (meth)acrylates—and also unreacted (meth)acrylic acid and unreacted polyhydroxy compounds—may enter into secondary reactions such as Michael additions with the fully (meth)acrylated species of substance, and also with one another.
